This invention relates to the field of waste recovery. More particularly, it relates to the field of waste treatment wherein energy is recovered from organic wastes, such as animal manure, sludges or food processing wastes. Such wastes contain an enormous energy value if converted to useable form. It has been suggested in the prior art that such wastes can be digested by enzymes and bacteria to produce methane gas and/or by-products. The methane gas can then be utilized to drive a generator set to produce electricity or directly utilized to operate gas fired furnaces or other machinery.
A principal problem in producing energy from organic waste is the difficulty in handling the waste slurry and the cost of maintaining ideal digesting conditions to obtain satisfactory yields of gas. Many prior attempts have required elaborate plumbing and pumping systems as well as the use of mechanical agitators in order to obtain effective digesting of the organic slurries. Often the initial costs are prohibitive and the net energy gain of these complex systems has been too small to justify practical use.
A further problem with proposed systems has been the formation of thick surface scum which effectively blocks the release of gas from the slurry into the collection bubble and adversely affects the generating process itself. Also, the digested solids collect along the bottom of the tank requiring frequent draining and cleaning. These problems further reduce the efficiency of the system and, under certain circumstances, can stop gas generation altogether.
It is accordingly an object of the present invention to provide an improved digester system for organic wastes.
It is a further object of the present invention to provide a digester system including a scum suppression mechanism which will prevent the formation of scum on the surface of the slurry.
A further object of the present invention is to provide a digester system of the type described which dispenses with the need for mechanical agitation to mix the slurry. According to the present invention, stirring of the slurry is attained by use of an improved heat exchanger located in the digester tank.
A further object of the invention is to provide means whereby solids which settle to the bottom of the tank can be periodically removed therefrom without the need for interrupting the operation of the digester or of draining the tank.